This only works if you are always scrupulous about it. If you remember to ask if they are wearing gloves whenever they propose to touch something, then chances are by the time that we get to the point where there is contact poison on the door knob you don't even need to prompt the player - they'll tell you about the gloves or whether they take them off when they do it.
But if you only bring it up when there is contact poison on the door, then of course they are wearing gloves and you need not bother to ask.